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  • Journal article
    Zhao Y, Patel Y, Hunt IA, Kareh KM, Holland AA, Korte C, Dear JP, Yan Y, Offer GJet al., 2017,

    Preventing lithium ion battery failure during high temperatures by externally applied compression

    , Journal of Energy Storage, Vol: 13, Pages: 296-303, ISSN: 2352-152X

    Lithium-ion cells can unintentionally be exposed to temperatures outside manufacturers recommended limits without triggering a full thermal runaway event. The question addressed in this paper is: Are these cells still safe to use? In this study, externally applied compression has been employed to prevent lithium ion battery failure during such events. Commercially available cells with Nickel Cobalt Manganese (NCM) cathodes were exposed to temperatures at 80 °C, 90 °C and 100 °C for 10 h, and electrochemically characterised before and after heating. The electrode stack structures were also examined using x-ray computed tomography (CT), and post-mortems were conducted to examine the electrode stack structure and surface changes. The results show that compression reduces capacity loss by −0.07%, 4.95% and 13.10% respectively, measured immediately after the thermal testing. The uncompressed cells at 80 °C showed no swelling, whilst 90 °C and 100 °C showed significant swelling. The X-ray CT showed that the uncompressed cell at 100 °C suffered de-lamination at multiple locations after test, and precipitations were found on the electrode surface. The post-mortem results indicates the compressed cell at 100 °C was kept tightly packed, and the electrode surface was uniform. The conclusion is that externally applied compression reduces delamination due to gas generation during high temperature excursions.

  • Book chapter
    Wu B, Offer G, 2017,

    Environmental impact of hybrid and electric vehicles

    , Environmental Impacts of Road Vehicles : Past, Present and Future, Editors: Harrison, Hester, Publisher: Royal Society of Chemistry

    Hybrid and electric vehicles play a critical role in reducing global greenhouse gas emissions, with transport estimated to contribute to 14% of the 49 GtCO2eq produced annually. Analysis of only the conversion efficiency of powertrain technologies can be misleading, with pure battery electric and hybrid vehicles reporting average efficiencies of 92% and 35% in comparison with 21% for internal combustion engine vehicles. A fairer comparison would be to consider the well-to-wheel efficiency, which reduces the numbers to 21–67%, 25% and 12%, respectively. The large variation in well-to-wheel efficiency of pure battery electric vehicles highlights the importance of renewable energy generation in order to achieve true environmental benefits. When calculating the energy return on investment of the various technologies based on the current energy generation mix, hybrid vehicles show the greatest environmental benefits, although this would change if electricity was made with high amounts of renewables. In an extreme scenario with heavy coal generation, the CO2eq return on investment can actually be negative for pure electric vehicles, highlighting the importance of renewable energy generation further. The energy impact of production is generally small (∼6% of lifetime energy) and, similarly, recycling is of a comparable magnitude, but it is less well studied.

  • Journal article
    Zhang T, Marinescu M, Walus S, Kovacik P, Offer GJet al., 2017,

    What Limits the Rate Capability of Li-S Batteries during Discharge: Charge Transfer or Mass Transfer?

    , Journal of the Electrochemical Society, Vol: 165, Pages: A6001-A6004, ISSN: 0013-4651

    Li-S batteries exhibit poor rate capability under lean electrolyte conditions required for achieving high practical energy densities. In this contribution, we argue that the rate capability of commercially-viable Li-S batteries is mainly limited by mass transfer rather than charge transfer during discharge. We first present experimental evidence showing that the charge-transfer resistance of Li-S batteries and hence the cathode surface covered by Li2S are proportional to the state-of-charge (SoC) and not to the current, directly contradicting previous theories. We further demonstrate that the observed Li-S behaviors for different discharge rates are qualitatively captured by a zero-dimensional Li-S model with transport-limited reaction currents. This is the first Li-S model to also reproduce the characteristic overshoot in voltage at the beginning of charge, suggesting its cause is the increase in charge transfer resistance brought by Li2S precipitation.

  • Journal article
    Walus S, Offer GJ, Hunt I, Patel Y, Stockley T, Williams J, Purkayastha Ret al., 2017,

    Volumetric expansion of Lithium-Sulfur cell during operation – Fundamental insight into applicable characteristics

    , Energy Storage Materials, Vol: 10, Pages: 233-245, ISSN: 2405-8297

    During the operation of a Lithium-Sulfur (Li-S) cell, structural changes take place within both positive and negative electrodes. During discharge, the sulfur cathode expands as solid products (mainly Li2S or Li2S/Li2S2) are precipitated on its surface, whereas metallic Li anode contracts due to Li oxidation/stripping. The opposite processes occur during charge, where Li anode tends to expand due to lithium plating and solid precipitates from the cathode side are removed, causing its thickness to decrease. Most research literature describe these processes as they occur within single electrode cell constructions. Since a large format Li-S pouch cell is composed of multiple layers of electrodes stacked together, and antagonistic effects (i.e. expansion and shrinkage) occur simultaneously during both charge and discharge, it is important to investigate the volumetric changes of a complete cell. Herein, we report for the first time the thickness variation of a Li-S pouch cell prototype. In these studies we used a laser gauge for monitoring the cell thickness variation under operation. The effects of different voltage windows as well as discharge regimes are explored. It was found that the thickness evolution of a complete pouch cell is mostly governed by Li anodes volume changes, which mask the response of the sulfur cathodes. Interesting findings on cell swelling when cycled at slow currents and full voltage windows are presented. A correlation between capacity retention and cell thickness variation is demonstrated, which could be potentially incorporated into Battery Management System (BMS) design for Li-S batteries.

  • Journal article
    Zhang X-F, Zhao Y, Patel Y, Zhang T, Liu W-M, Chen M, Offer GJ, Yan Yet al., 2017,

    Potentiometric measurement of entropy change for lithium batteries

    , PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 19, Pages: 9833-9842, ISSN: 1463-9076
  • Journal article
    Hunt IA, Patel Y, Szczygielski M, Kabacik L, Offer GJet al., 2015,

    Lithium Sulfur battery nail penetration test under load

    , Journal of Energy Storage

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